Communications across a network address translator

ABSTRACT

A method, apparatus, and computer-readable media are presented that provide a configuration for communications through network address translation. The configuration includes receiving, by a computer device, a packet comprising a predetermined value indicating support by a node for an extension of a communications protocol, wherein the communications protocol is used for communications across a network translator device and the extension is capable of traversing network address translation, and in response to said receiving, determining that the node sending the packet supports the extension of the communications protocol.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.12/862,305, filed Aug. 24, 2010, now U.S. Pat. No. 8,544,079, which is acontinuation of U.S. application Ser. No. 11/128,933, filed May 12,2005, now U.S. Pat. No. 8,127,348, which is a continuation of U.S.application Ser. No. 09/333,829, filed Jun. 15, 1999, now U.S. Pat. No.6,957,346. The entire contents of all applications are incorporatedherein by reference in their entireties.

TECHNOLOGICAL FIELD

The invention relates in general to the field of communications betweencomputers in packet-switched data transmission networks. Moreparticularly the invention relates to the field of communicationconnections through a Network Address Translation.

BACKGROUND OF THE INVENTION

The Internet Engineering Task Force (IETF) has standardized the IPSEC(Internet Protocol Security) protocol suite; the standards are wellknown from the Request For Comments or RFC documents number RFC2401,RFC2402, RFC2406, RFC2407, RFC2408 and RFC2409 mentioned in the appendedlist of references, all of which are hereby incorporated by reference.The IPSEC protocols provide security for the IP or Internet Protocol,which itself has been specified in the RFC document number RFC791.

IPSEC performs authentication and encryption on packet level bygenerating a new IP header, adding an Authentication Header (AH) orEncapsulating Security Payload (ESP) header in front of the packet. Theoriginal packet is cryptographically authenticated and optionallyencrypted. The method used to authenticate and possibly encrypt a packetis identified by a security parameter index (SPI) value stored in the AHand ESP headers. The RFC document number RFC2401 specifies a transportmode and a tunnelling mode for packets; the present invention isapplicable regardless of which of these modes is used.

In recent years, more and more vendors and Internet service providershave started performing network address translation (NAT). References toNAT are found at least in the RFC document number RFC1631 as well as thedocuments which are identified in the appended list of references asSrisuresh98Terminology, SrisureshEgevang98, Srisuresh98Security,HoldregeSrisuresh99, TYS99, Rekhter99, LoBorella99 and BorellaLo99.There are two main forms of address translation, illustratedschematically in FIGS. 1 a and 1 b: host NAT 101 and port NAT 151. HostNAT 101 only translates the IP addresses in an incoming packet 102 sothat an outgoing packet 103 has a different IP address. Port NAT 151also touches the TCP and UDP port numbers (Traffic Control Protocol;User Datagram Protocol) in an incoming packet 152, multiplexing severalIP addresses to a single IP address in an outgoing packet 153 andcorrespondingly demultiplexing a single IP address into several IPaddresses for packets travelling in the opposite direction (not shown).Port NATs are especially common in the home and small officeenvironment. The physical separation of input and output connections forthe NAT devices is only shown in FIGS. 1 a and 1 b for graphicalclarity; in practice there are many possible ways for physicallyconnecting a NAT.

Address translation is most frequently performed at the edge of a localnetwork (i.e., translation between multiple local private addresses onone hand and fewer globally routable public addresses on the other).Most often, port NAT is used and there is only one globally routableaddress. A local network 154 has been schematically illustrated in FIG.1 b. Such arrangements are becoming extremely commonplace in the homeand small office markets. Some Internet service providers have alsostarted giving private addresses to their customers, and perform addresstranslation in their core networks for such addresses. In general,network address translation has been widely discussed in depth e.g. inthe NAT working group within the Internet Engineering Task Force. Theoperating principles of a NAT device are well known, and there are manyimplementations available on the market from multiple vendors, includingseveral implementations in freely available source code. The typicaloperation of a NAT may be described so that it maps IP address and portcombinations to different IP address and port combinations. The mappingwill remain constant for the duration of a network connection, but maychange (slowly) with time. In practice, the NAT functionality is oftenintegrated into a firewall or a router.

FIG. 1 c illustrates an exemplary practical network communicationsituation where a transmitting node 181 is located in a first local areanetwork (also known as the first private network) 182, which has a portNAT 183 to connect it to a wide-area general packet-switched network 184like the Internet. The latter consists of a very large number of nodesinterconnected in an arbitrary way. A receiving node 185 is located in asecond local area network 186 which is again coupled to the wide-areanetwork through a NAT 187. The denominations “transmitting node” and“receiving node” are somewhat misleading, since the communicationrequired to set up network security services is bidirectional. Thetransmitting node is the one that initiates the communication. Also theterms “Initiator” and “Responder” are used for the transmitting node andthe receiving node respectively.

The purpose of FIG. 1 c is to emphasize the fact that the communicatingnodes are aware of neither the number or nature of the intermediatedevices through which they communicate nor the nature of transformationsthat take place. In addition to NATs, there are other types of deviceson the Internet that may legally modify packets as they are transmitted.A typical example is a protocol converter, whose main job is to convertthe packet to a different protocol without disturbing normal operation.Using them leads to problems very similar to the NAT case. A fairlysimple but important example is converting between IPv4 and IPv6, whichare different versions of the Internet Protocol. Such converters will beextremely important and commonplace in the near future. A packet mayundergo several conversions of this type during its travel, and it ispossible that the endpoints of the communication actually use adifferent protocol. Like NAT, protocol conversion is often performed inrouters and firewalls.

It is well known in the IPSEC community that the IPSEC protocol does notwork well across network address translations. The problem has beendiscussed at least in the references given as HoldregeSrisuresh99 andRekhter99.

In the Finnish patent application number 974665 and the correspondingPCT application number FI98/01032, which are incorporated herein byreference, we have presented a certain method for performing IPSECaddress translations and a method for packet authentication that isinsensitive to address transformations and protocol conversions en routeof the packet. Additionally in said applications we have presented atransmitting network device and a receiving network device that are ableto take advantage of the aforementioned method. However, some problemsrelated to the provision of network security services over networkaddress translation remain unsolved in said previous patentapplications.

SUMMARY OF THE INVENTION

It is an object of the present invention to present a method and thecorresponding devices for providing network services over networkaddress translation in a reliable and advantageous way.

According to a first aspect of the invention, there is provided a methodcomprising receiving, by a computer device, a packet comprising apredetermined value indicating support by a node for an extension of acommunications protocol, wherein the communications protocol is used forcommunications across a network translator device and the extension iscapable of traversing network address translation, and in response tosaid receiving, determining that the node sending the packet supportsthe extension of the communications protocol.

According to a second aspect of the invention, there is provided anapparatus comprising at least one processor, and at least one memoryincluding computer program code, the at least one memory and thecomputer program code being configured to, with the at least oneprocessor, receive a packet comprising a predetermined value indicatingsupport by a node for an extension of a communications protocol, whereinthe communications protocol is used for communications across a networktranslator device and the extension is capable of traversing networkaddress translation, and in response to said received packet, determinethat the node sending the packet supports the extension of thecommunications protocol.

According to a third aspect of the invention, there is providednon-transitory computer readable media, comprising program code programcode for causing a processor to perform instructions for receiving, by adevice, a packet comprising a predetermined value indicating support bya node for an extension of a communications protocol, wherein thecommunications protocol is used for communications across a networktranslator device and the extension is capable of traversing networkaddress translation, and in response to said receiving, determining thatthe node sending the packet supports the extension of the communicationsprotocol.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a illustrates the known use of a host NAT,

FIG. 1 b illustrates the known use of a port NAT,

FIG. 1 c illustrates a known communication connection between nodesthrough a packet-switched network,

FIG. 2 a illustrates a certain Vendor ID payload applicable within thecontext of the invention,

FIG. 2 b illustrates a certain private payload applicable within thecontext of the invention,

FIG. 2 c illustrates a certain combined header structure applicablewithin the context of the invention,

FIG. 3 illustrates certain method steps related to the application ofthe invention,

FIG. 4 illustrates a transformation of header structures according to anaspect of the invention, and

FIG. 5 illustrates a simplified block diagram of a network device usedto implement the method according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention combines and extends some of the methods ofnetwork address translation, tunneling over UDP, IKE, and the IKEextension mechanisms, in a novel and inventive way to produce a methodfor secure communications across network address translations andprotocol conversions. The method can be made fully automatic andtransparent to the user.

A key point relating to the applicability of the invention is that—atthe priority date of the present patent application—in general only TCP(described in RFC793, which is hereby incorporated by reference) and UDP(described in RFC768, which is hereby incorporated by reference) workover NAT. This is because most NATs used in practise are port NATs, andthis is the form of NAT that provides most benefits with regards to theshortage of globally routable IP addresses. The invention is not,however, limited to the use of UDP and TCP as they are known at thepriority date of this patent application: in general it may be said thatUDP and TCP are examples of protocols that determine that connectionidentification information (i.e. addressing and port numbering) that ismapped into another form in the address transformation process. We mayexpect that other kinds of communication protocols and addresstransformations emerge in the future.

The various aspects of the invention are related to

-   -   determining whether a remote host supports a certain method        which is typically a secure communication method according to        the invention (the “methods supported” aspect),    -   determining what network address translations and/or protocol        conversions occur on packets, if any (the “occurring        translations” aspect),    -   tunneling packets inside a certain carefully selected protocol,        typically UDP, to make them traverse NATs (the “selected        tunnelling” aspect),    -   using a keepalive method to make sure that involved NAT devices        and other devices that use timeouts for mappings do not lose the        mapping for the communicating hosts (the “keepalive” aspect),    -   compensating for the translations that occur before verifying        the message authentication code for AH packets (the        “compensation/authentication” aspect) and    -   performing address translations at either the sending or        receiving node to compensate for multiple hosts being mapped to        a single public address (the “compensation/mapping” aspect).

The process of encapsulating data packets for transmission over adifferent logical network is called tunneling. Typically, in the case ofthe IP protocol, tunneling involves adding a new IP header in front ofthe original packet, setting the protocol field in the new headerappropriately, and sending the packet to the desired destination(endpoint of the tunnel). Tunneling may also be implemented by modifyingthe original packet header fields or replacing them with a differentheader, as long as a sufficient amount of information about the originalpacket is saved in the process so that it will be possible toreconstruct the packet at the end of the tunnel into a form sufficientlysimilar to the original packet entering the tunnel. The exact amount ofinformation that needs to be passed with the packet depends on thenetwork protocols, and information may be passed either explicitly (aspart of the tunnelled packet) or implicitly (by the context, asdetermined e.g. by previously transmitted packets or a contextidentifier in the tunneled packet).

It is well known in the art how to tunnel packets over a network. Atleast the references given as RFC1226, RFC1234, RFC1241, RFC1326,RFC1701, RFC1853, RFC2003, RFC2004, RFC2107, RFC2344, RFC2401, RFC2406,RFC2473 and RFC2529 (all of which are hereby incorporated by reference)relate to the subject of tunneling. For example, RFC1234 presents amethod of tunneling IPX frames over UDP. In that method, packets aretunneled to a fixed UDP port and to the decapsulator's IP address.

The IPSEC protocol mentioned in the background description typicallyuses the Internet Key Exchange or IKE protocol (known from referencesRFC2409, RFC2408 and RFC2407, all of which are hereby incorporated byreference) for authenticating the communicating parties to each other,deriving a shared secret known only to the communicating parties,negotiating authentication and encryption methods to be used for thecommunication, and agreeing on a security parameter index (SPI) valueand a set of selectors to be used for the communication. The IKEprotocol was previously known as the ISAKMP/Oakley, where the acronymISAKMP comes from Internet Security Association Key Management Protocol.Besides said normal negotiation specified in the IKE standard, IKEsupports certain mechanisms for extension. The Vendor ID payload knownfrom reference RFC2408, which is hereby incorporated by reference,allows communicating parties to determine whether the other partysupports a particular private extension mechanism. The IPSEC DOI (Domainof Interpretation) known as RFC2407, which is hereby incorporated byreference, reserves certain numeric values for such private extensions.

Currently, the well-known Vendor ID payload is defined to have theformat illustrated in FIG. 2 a, where the column numbers correspond tobit positions.

For the purposes of the present invention the Vendor ID field 201 is themost important part of the Vendor ID payload. In the context of the IKEprotocol, negotiating whether the remote host supports a certain methodfor providing secure network communications can be performed as follows.The terminology used here is borrowed from the IKE documents.

The IKE protocol determines the so-called Phase 1 of the mutual exchangeof messages between the Initiator (i.e., the node first sending a packetto the other) and the Responder (i.e., the node first receiving apacket). FIG. 3 illustrates an exchange of first Phase 1 messagesbetween the Initiator and the Responder. According to the “methodssupported” aspect of the invention both devices include a certain VendorID Payload in a certain Phase 1 message which is most advantageouslytheir first Phase 1 message. This payload indicates that they supportthe method in question.

In FIG. 3 the Vendor ID fields contained within the Initiator's first(or other) Phase 1 message is schematically shown as 201′ and the VendorID fields contained within the Responder's first (or other) Phase 1message is schematically shown as 201″. To indicate support for acertain method the Vendor ID field in the Vendor ID Payload is basicallyan identification of that method: advantageously it is the MD5 hash of apreviously known identification string, e.g. “SSH IPSEC NAT TraversalVersion 1”, without any trailing zeroes or newlines. Producing MD5hashes of arbitrary character sequences is a technique well known in theart for example from the publication RFC1321, which is herebyincorporated by reference, mentioned in the list of references.

Next we will address the “occurring translations” aspect of theinvention. In addition to the above-mentioned Phase 1, the IKE protocoldetermines the so-called Phase 2 of the mutual exchange of messagesbetween the Initiator and the Responder. According to the “occurringtranslations” aspect of the invention the parties can determine whichtranslations occur by including the IP addresses they see in privatepayloads of certain Phase 2 Quick Mode messages, which are mostadvantageously their first Phase 2 Quick Mode messages. Any unusednumber in the private payload number range can be used to signify suchuse of the private payload (e.g. 157, which is unused at the prioritydate of the present patent application).

The private payload used to reveal the occurring translations can havee.g. the format illustrated in FIG. 2 b. Field 211 contains a type codethat identifies the types of the addresses that appear in fields 212 and213. Field 212 contains the address of the Initiator as seen by the nodesending the message, and field 213 contains the address of the Responderas seen by the node sending the message. FIG. 3 shows the exchange of(first) Phase 2 Quick Mode messages between the Initiator and theResponder so that the corresponding fields 211′, 212′ and 213′ areincluded in the message sent by the former and the fields 211″, 212″ and213″ are included in the message sent by the latter.

According to known practice the addresses of the Initiator and Responderare also included in the header of the packet that contains the payloadof FIG. 2 b. In the header they are susceptible to address translationsand other processing whereas in the private payload they are not. Whenthe packet with the payload of FIG. 2 b is received, the addressescontained in it are compared with those seen in the packet header. Ifthey differ, then an address translation occurred on the packet. Laterwe will refer to the use of the standard IKE port number 500 togetherwith applying the invention; as an additional way of detecting occurredtranslations the port numbers of the received packet can also becompared against the standard IKE port number 500 to determine if porttranslations occurred.

An aspect of some importance when handling the addresses is that the UDPsource port of the packet can be saved for later use. It would usuallybe saved with the data structures for Phase 1 ISAKMP securityassociations, and would be used to set up compensation processing forPhase 2 IPSEC security associations.

To use the method described above to implement the “occurredtranslations” aspect of the invention, the hosts must modify their Phase2 identification payloads: the payload illustrated in FIG. 2 b is notknown in the existing standards. One possibility is to restrict thepayloads to the ID_IPV4_ADDR and ID_IPV6_ADDR types, which would beappropriate for host-to-host operation.

Next we will address the “selected tunnelling”,“compensation/authentication” and “compensation/mapping” aspects of theinvention. According to this aspect of the invention the actual datapackets can be tunneled over the same connection which is used to set upthe security features of the communication connection, e.g. the UDPconnection used for IKE. This ensures that the actual data packets willexperience the same translations as the IKE packets did when thetranslation was determined. Taken that the standard port number 500 hasbeen determined for IKE, this would mean that all packets are sent withsource port 500 and destination port 500, and a method is needed todistinguish the real IKE packets from those containing encapsulateddata. One possible way of doing this takes advantage of the fact thatthe IKE header used for real IKE packets contains an Initiator Cookiefield: we may specify that Initiators that support this aspect of theinvention never generate cookies that have all zeroes in their fourfirst bytes. The value zero in the corresponding four bytes is then usedto recognize the packet as a tunneled data packet. In this way, tunneleddata packets would have four zero bytes at the beginning of the UDPpayload, whereas real IKE packets never would.

FIG. 4 illustrates the encapsulation of actual IPSEC packets into UDPfor transmission. Basically, a UDP header 403 and a short intermediateheader 404 are inserted after the IP header 401 already in the packet(with the protocol field copied to the intermediate header). The IPheader 401 is slightly modified to produce a modified IP header 401′.The IP payload 402 stays the same. The simple illustration of theunencapsulated IPSEC packet on the left should not be misinterpreted:this packet is not plaintext but has been processed according to AH orESP or corresponding other transformation protocol in the sending nodebefore its encapsulation into UDP.

Without limiting the generality, it is assumed in the presentation herethat the encapsulation according to FIG. 4 is always performed by thesame nodes that perform IPSEC processing (either an end node or a VPNdevice). It should also be noted that instead of encapsulating the IPSECpackets into UDP they could be encapsulated into TCP. This alternativewould probably require using fake session starts and ends so that thefirst packet has the SYN bit and the last packet has the FIN bit, asspecified in the TCP protocol.

In encapsulating an actual data packet or a “datagram” according to FIG.4, the original IP header 401—defined in RFC791, which is herebyincorporated by reference,—is modified to produce the modified IP header401′ as follows:

-   -   the Protocol field in the IP header (not separately shown) is        replaced by protocol 17 for UDP in accordance with RFC768, which        is hereby incorporated by reference,    -   the Total Length field in the IP header (not separately shown)        is incremented by the combined size of the UDP and intermediate        headers (total 16 bytes) and    -   the Header Checksum field in the IP header (not separately        shown) is recomputed in accordance with the rules given in        RFC791, which is hereby incorporated by reference.

As seen from FIG. 4, an UDP header 403—as defined in RFC768, which ishereby incorporated by reference,—and an intermediate header 404 areinserted after the IP header. The UDP header is 8 octets and theintermediate header is 8 octets, for a total of 16 octets. These headersare treated as one in the following discussion. The combined header hasmost advantageously the format illustrated in FIG. 2 c. Fields of thisheader are set as follows:

-   -   The Source Port field 221 is set to 500 (same as IKE). If the        packet goes through NAT, this may be different when the packet        is received.    -   The Destination Port field 222 is set to the port number from        which the other end appears to be sending packets. If the packet        goes through NAT, the recipient may see a different port number        here.    -   The UDP Length field 223 is the length of the UDP header plus        the length of the UDP data field. In this case, it also includes        the intermediate header. The value is computed in bytes as 16        plus the length of the original IP packet payload (not including        the original IP header, which is included in the Length field in        the IP header).    -   The UDP Checksum field 224 is most advantageously set to 0. The        UDP checksum is optional, and we do not wish to calculate or        check it with this tunneling mechanism. Integrity of the data is        assumed to be protected by an AH or ESP header within the        tunneled packet.    -   The Must be zero field 225: This field must contain a previously        agreed fixed value, which is most advantageously all zeroes. The        field overlaps with the first four bytes of the Initiator Cookie        field in an actual IKE header. Any Initiator that supports this        aspect of the invention must not use a cookie where the first        four bytes are zero. These zero bytes are used to separate the        tunneled packets from real ISAKMP packets. Naturally some other        fixed value than “all zeroes” could be chosen, but the value        must be fixed for this particular use.    -   Protocol field 226: The value of this field is copied from the        known Protocol field in the original IP header (not separately        shown in FIG. 4).    -   Reserved field 227: most advantageously sent as all zeroes;        ignored on reception.

The sender inserts this header in any packets tunneled to a destinationbehind NAT. Information about whether NAT is used can be stored on a perSA (Security Association) basis in the policy manager. The encapsulationreferred to in FIG. 4 can be implemented either as a new transform or aspart of the otherwise known AH and ESP transforms.

The encapsulation operation makes use of the UDP port number and IPaddress of the remote host, which were determined during the IKEnegotiation.

The receiver decapsulates packets from this encapsulation before doingAH or ESP processing. Decapsulation removes this header and updates theProtocol, Length, and Checksum fields of the IP header. No configurationdata (port number etc.) is needed for this operation.

The decapsulation should be performed only if all of the followingselectors match:

-   -   destination address is the destination address of this host,    -   source address is the address of a host with which this host has        agreed to use this tunnelling,    -   the Protocol field indicates UDP,    -   the Destination port field value is 500 and    -   the Source port field value indicates the port with which this        host has agreed to use this tunneling. (Note that there may be        multiple source addresses and ports for which this tunneling is        performed; each of them is treated by a separate set of        selectors.)

During decapsulation the source address in the received packet can bereplaced by the real source address received during the IKE negotiation.This implements the compensation for AH MAC verification. The address isagain changed in the post-processing phase below. Because of thiscompensation, the standard AH and ESP transforms can be used unmodified.

In FIG. 3 the AH/ESP processing at the sending node is schematicallyshown as block 301, encapsulation of datagrams into UDP is schematicallyshown as block 302, the corresponding decapsulation of datagrams fromUDP is schematically shown as block 303 and AH/ESP processing at thereceiving node is schematically shown as block 304.

Additional compensation must be done after the packet has beendecapsulated from AH or ESP. This additional decapsulation must dealwith the fact that the outer packet actually went through NAT(illustrated schematically in FIG. 3 as block 305), and consequently theplaintext packet must also undergo a similar transformation. Therecipient must see the address of the NAT device as the address of thehost, rather than the original internal address. Alternatively, thiscompensation could have been performed by the sender of the packetbefore encapsulating it within AH or ESP.

There are several alternatives for this additional compensation forvarious special cases (the best compensation depends on the particularapplication):

-   -   Allocating a range of network addresses for this processing        (say, in the link-local use range 169.254.x.x—the actual values        do not matter; basically we just want an arbitrary network that        no-one else is using). An address in this range is allocated for        each <natip, ownip, natport, ownport> combination, where natip        means the IP address of the NAT, ownip means the processing        device's own IP address, natport means the port number at the        NAT and own port means the processing device's own port number.        The remote address in the packet is replaced by this address        before the packet is sent to protocol stacks.    -   As part of the compensation, the TCP checksum for internal hosts        must be recomputed if host addresses or port numbers changed.        TCP checksum computations may also be incremental, as is known        from RFC1071, which is hereby incorporated by reference. Port        NAT may need to be performed for the source port.    -   When used as a VPN between two sites using incompatible        (possibly overlapping) private address spaces, address        translation must be performed to make the addresses compatible        with local addresses.    -   When used as a VPN between two sites using compatible        (non-overlapping) private address spaces, and tunnel mode is        used, no additional compensation may be needed.    -   Address translation may need to be performed for the contents of        certain protocol packets, such as FTP (known from RFC959, which        is hereby incorporated by reference) or H.323. Other similar        issues are discussed in the reference given as        HoldregeSrisuresh99.    -   It may also be possible to use random addresses for the client        at the server, and perform address translation to this address.        This could allow the server to distinguish between multiple        clients behind the same NAT, and could avoid manual        configuration of the local address space.    -   The compensation operation may or may not interact with the        TCP/IP stack on the local machine to reserve UDP port numbers.

In general, this invention does not significantly constrain the methodused to compensate for inner packets the NAT occurring for the outerheader. The optimal method for performing such compensation may be foundamong the above-given alternatives by experimenting, or some otheroptimal method could be presented.

Next we will address the “keepalive” aspect of the invention, i.e.ensuring that the network address translations performed in the networkdo not change after the translations that occur have been determined.Network address translators cache the information about address mapping,so that they can reverse the mapping for reply packets. If TCP is used,the address translator may look at the FIN bit of the TCP header todetermine when it can drop a particular mapping. For UDP, however, thereis no explicit termination indication for flows. For this reason, manyNATs will time out mappings for UDP quite fast (even as fast as in 30seconds). Thus, it becomes necessary to force the mapping to bemaintained.

A possible way of ensuring the maintaining of mappings is to sendkeepalive packets frequently enough that the address translation remainsin the cache. When computing the required frequency, one must take intoaccount that packets may be lost in the network, and thus multiplekeepalives must be sent within the estimated shortest period in whichNATs may forget the mapping. The appropriate frequency depends on boththe period the mappings are kept cached and on the packet lossprobability of the network; optimal frequency values for various contextmay be found through experimenting.

Keepalive packets do not need to contain any meaningful informationother than the necessary headers that are equal to the data packetheaders to ensure that the keepalive packets will be handled exactly inthe same way as the actual data packets. A keepalive packet may containan indicator that identifies it as a keepalive packet and not a datapacket; however it may also be determined that all packets that do notcontain meaningful payload information are interpreted to be keepalivepackets. In FIG. 3 the transmission of keepalive packets isschematically illustrated by block 306 and the reception and discardingof them is schematically illustrated by block 307. It should be notedthat the use of keepalive packets is not needed at all if actual datapackets are transmitted frequently enough and/or the connection is toremain valid only for such a short time (e.g. a few seconds) that it isimprobable that any intermediate device would delete the mappinginformation from its cache. Keepalive packets need to be transmitted inone direction only, although they may be transmitted alsobidirectionally; the drawback resulting from their bidirectionaltransmission is the resulting increase in unnecessary network traffic.The invention does not limit the direction(s) in which keepalive packets(if any) are transmitted.

FIG. 5 is a simplified block diagram of a network device 500 that canact as the Initiator or the Responder according to the method ofproviding secure communications over network address translations inaccordance with the invention. Network interface 501 connects thenetwork device 500 physically to the network. Address management block502 keeps track of the correct network addresses, port numbers and otheressential public identification information of both the network device500 itself and its peer (not shown). IKE block 503 is responsible forthe key management process and other activities related to the exchangeof secret information.

Encryption/decryption block 504 implements the encryption and decryptionof data once the secret key has been obtained by the IKE block 503.Compensation block 505 is used to compensate for the permissibletransformations in the transmitted and/or received packets according tothe invention. Either one of blocks 504 and 505 may be used to transmit,receive and discard keepalive packets. Packet assembler/disassemblerblock 506 is the intermediator between blocks 502 to 505 and thephysical network interface 501. All blocks operate under the supervisionof a control block 507 which also takes care of the routing ofinformation between the other blocks and the rest of the network device,for example for displaying information to the user through a displayunit (not shown) and obtaining commands from the user through a keyboard(not shown). The blocks of FIG. 5 are most advantageously implemented aspre-programmed operational procedures of a microprocessor, whichimplementation is known as such to the person skilled in the art. Otherarrangements than that shown in FIG. 5 may as well be used to reduce theinvention into practice.

Even though the present invention was presented in the context of IKE,and tunneling using the IKE port, it should be understood that theinvention applies to also other analogous cases using different packetformatting methods, different negotiation details, a different keyexchange protocol, or a different security protocol. The invention mayalso be applicable to non-IP protocols with suitable characteristics.The invention is equally applicable to both IPv4 and IPv6 protocols. Theinvention is also intended to apply to future revisions of the IPSEC andIKE protocols.

It should also be understood that the invention can also be applied toprotocol translations in addition to just address translations. Adaptingthe present invention to protocol translations should be well within thecapabilities of a person skilled in the art given the description hereand the discussions regarding protocol translation in the former patentapplications of the same applicant mentioned above and incorporatedherein by reference.

List of References

All of the following references are hereby incorporated by reference.

-   BorellaLo99-   M. Borella, J. Lo: Realm Specific IP: Protocol Specification,    draft-ietf-nat-rsip-protocol-00.txt, Work in Progress, Internet    Engineering Task Force, 1999.

HoldregeSrisuresh99

-   M. Holdrege, P. Srisuresh: Protocol Complications with the IP    Network Address Translator (NAT),    draft-ietf-nat-protocol-complications-00.txt, Work in Progress,    Internet Engineering Task Force, 1999.

LoBorella99

-   J. Lo, M. Borella: Real Specific IP: A Framework,    draft-ietf-nat-rsip-framework-00.txt, Work in Progress, Internet    Engineering Task Force, 1999.

Rekhter99

-   Y. Rekhter: Implications of NATs on the TCP/IP architecture,    draft-ietf-nat-arch-implications-00.txt, Internet Engineering Task    Force, 1999.

RFC768

-   J. Postel: User Datagram Protocol, RFC 768, Internet Engineering    Task Force, 1980.

RFC791

-   J. Postel: Internet Protocol, RFC 791, Internet Engineering Task    Force, 1981.

RFC793

-   J. Postel: Transmission Control Protocol, RFC 793, Internet    Engineering Task Force, 1981.

RFC959

-   J. Postel, J. Reynolds: File Transfer Protocol, RFC 959, Internet    Engineering Task Force, 1985.

RFC1071

-   R. Braden, D. Borman, C. Partridge: Computing the Internet checksum,    RFC 1071, Internet Engineering Task Force, 1988.

RFC1226

-   B. Kantor: Internet protocol encapsulation of AX.25 frames, RFC    1226, Internet Engineering Task Force, 1991.

RFC1234

-   D. Provan: Tunneling IPX traffic through IP networks, RFC 1234,    Internet Engineering Task Force, 1991.

RFC1241

-   R. Woodburn, D. Mills: Scheme for an internet encapsulation    protocol: Version 1, RFC 1241, Internet Engineering Task Force,    1991.

RFC1321

-   R. Rivest: The MD5 message-digest algorithm, RFC 1321, Internet    Engineering Task Force, 1992.

RFC1326

-   P. Tsuchiya: Mutual Encapsulation Considered Dangerous, RFC 1326,    Internet Engineering Task Force, 1992.

RFC1631

-   K. Egevang, P. Francis: The IP Network Address Translator (NAT), RFC    1631, Internet Engineering Task Force, 1994.

RFC1701

-   S. Hanks, T. Li, D. Farinacci, P. Traina: Generic Routing    Encapsulation, RFC 1701, Internet Engineering Task Force, 1994.

RFC1702

-   S. Hanks, T. Li, D. Farinacci, P. Traina: Generic Routing    Encapsulation over IPv4 networks, RFC 1702, Internet Engineering    Task Force, 1994.

RFC1853

-   W. Simpson: IP in IP Tunneling, RFC 1853, Internet Engineering Task    Force, 1995.

RFC2003

-   C. Perkins: IP Encapsulation within IP, RFC 2003, Internet    Engineering Task Force, 1996.

RFC2004

-   C. Perkins: IP Encapsulation within IP, RFC 2004, Internet    Engineering Task Force, 1996.

RFC2107

-   K. Hamzeh: Ascend Tunnel Management Protocol, RFC 2107, Internet    Engineering Task Force, 1997.

RFC2344

-   G. Montenegro: Reverse Tunneling for Mobile IP, FC 2344, Internet    Engineering Task Force, 1998.

RFC2391

-   P. Srisuresh, D. Gan: Load Sharing using IP Network Address    Translation (LSNAT), RFC 2391, Internet Engineering Task Force,    1998.

RFC2401

-   S. Kent, R. Atkinson: Security Architecture for the Internet    Protocol, RFC 2401, Internet Engineering Task Force, 1998.

RFC2402

-   S. Kent, R. Atkinson: IP Authentication Header, RFC 2402, Internet    Engineering Task Force, 1998.

RFC2406

-   S. Kent, R. Atkinson: IP Encapsulating Security Payload, RFC 2406,    Internet Engineering Task Force, 1998.

RFC2407

-   D. Piper: The Internet IP Security Domain of Interpretation for    ISAKMP. RFC 2407, Internet Engineering Task Force, 1998.

RFC2408

-   D. Maughan, M. Schertler, M. Schneider, J. Turner: Internet Security    Association and Key Management Protocol (ISAKMP), RFC 2408, Internet    Engineering Task Force, 1998.

RFC2409

-   D. Hakins, D. Carrel: The Internet Key Exchange (IKE), RFC 2409,    Internet Engineering Task Force, 1998.

RFC2473

-   A. Conta, S. Deering: Generic Packet Tunneling in IPv6    Specification, RFC 2473, Internet Engineering Task Force, 1998.

RFC2529

-   B. Carpenter, C. Jung: Transmission of IPv6 over IPv4 Domains    without Explicit Tunnels, RFC 2529, Internet Engineering Task Force,    1999.

Srisuresh98Terminology

-   P. Srisuresh: IP Network Address Translator (NAT) Terminology and    Considerations, draft-ietf-nat-terminology-01.txt, Work in Progress,    Internet Engineering Task Force, 1998.

Srisuresh98Security

-   P. Srisuresh: Security Model for Network Address Translator (NAT)    Domains, draft-ietf-nat-security-01.txt, Work in Progress, Internet    Engineering Task Force, 1998.

SrisureshEgevang98

-   P. Srisuresh, K. Egevang: Traditional IP Network Address Translator    (Traditional NAT), draft-ietf-nat-traditional-01.txt, Work in    Progress, Internet Engineering Task Force, 1998.

TYS99

-   W. Teo, S. Yeow, R. Singh: IP Relocation through twice Network    Address Translators (RAT), draft-ietf-nat-rnat-00.txt, Work in    Progress, Internet Engineering Task Force, 1999.

The invention claimed is:
 1. A method comprising: receiving, by acomputer device, a packet comprising a predetermined value indicatingsupport by a node for an extension of a communications protocol, whereinthe communications protocol is used for communications across a networktranslator device and the extension is capable of traversing networkaddress translation, and in response to said receiving, determining thatthe node sending the packet supports the extension of the communicationsprotocol.
 2. The method according to claim 1, wherein the communicationsprotocol is a secure communications protocol.
 3. The method according toclaim 1, further comprising setting up a communications connectionbetween the computer device and the node in response to saiddetermining.
 4. An apparatus comprising at least one processor, and atleast one memory including computer program code, the at least onememory and the computer program code being configured to, with the atleast one processor, receive a packet comprising a predetermined valueindicating support by a node for an extension of a communicationsprotocol, wherein the communications protocol is used for communicationsacross a network translator device and the extension is capable oftraversing network address translation, and in response to said receivedpacket, determine that the node sending the packet supports theextension of the communications protocol.
 5. The apparatus according toclaim 4, wherein the communications protocol is a secure communicationsprotocol.
 6. The apparatus according to claim 4, further configured toset up a communications connection with the node in response to saiddetermining.
 7. A non-transitory computer readable media, comprisingprogram code for causing a processor to perform instructions forreceiving, by a device, a packet comprising a predetermined valueindicating support by a node for an extension of a communicationsprotocol, wherein the communications protocol is used for communicationsacross a network translator device and the extension is capable oftraversing network address translation, and in response to saidreceiving, determining that the node sending the packet supports theextension of the communications protocol.
 8. The non-transitory computerreadable media according to claim 7, wherein the communications protocolis a secure communications protocol.
 9. The non-transitory computerreadable media according to claim 7, further causing setting up acommunications connection between the device and the node in response tosaid determining.